It was half a century ago, but David Sheehan still vividly remembers the signs on the entry doors.

One was for black people, the other whites. Sheehan, who was 8 years old and on a trip to Washington, D.C., asked his stepmother why there were separate entrances.

“At a loss for words, she said, ‘That’s how things are here, and we’ll be back home in Massachusetts soon,’ ” said Sheehan.

Sheehan has made it his life’s work to answer questions like that. He brings the world to Watertown as the high school’s social studies curriculum coordinator.

So far this year he has invited Medal of Honor winners from World War II and a Japanese survivor of the atomic bomb.

Sheehan snared the war heroes when they were in Boston for a national convention. The A-bomb survivor was among members of the Japan Council against Atomic and Hydrogen Bombs , whose other US stops included Harvard and the United Nations.

“You have to get young people to make direct contact . . . in order to understand what they think,” said Sheehan. “The core of students’ learning is being energized by reality and not just in ways that are stymied by standardized testing.”

Sheehan has been energizing students in Watertown schools for 35 years .

In 1990, he received a grant from the Japanese Diet to study Japanese education for a month. When he returned to Watertown, he arranged for a weekly satellite hookup between a Japanese high school and nearly 50 American high schools. Sheehan said the students found that they had a lot in common, from pop culture to financial concerns.

“They also got to see how technology could be brought into the classroom and transcend national borders,” said Sheehan. The program lasted three years.

In 1995, Sheehan journeyed to China through a Fulbright scholarship. When he returned, he developed a curriculum with the English department to teach about the Tiananmen Square protests through literature and history.

In 2001, Sheehan toured India and Nepal on another Fulbright scholarship.

“I traveled by elephant, truck, motorcycle, boat, train, plane , and car to go from the far desert in the northwest to the southeast city of Chennai [India],” he said.

Sheehan arrived in Nepal two weeks after its royal family was assassinated.

“It was very touchy going into Katmandu because we weren’t sure if the Maoist movement was operating in the hills, ” he said.

When he returned to Watertown, he developed a unit comparing the civil rights movements in India and the United States.

Sheehan, 57, graduated from Watertown High School in 19 67 . He attended Clark University and earn ed a law degree and a master’s in Russian history from Boston University at night.

With his reddish blond hair neatly parted, and wearing a brown tweed blazer, Sheehan looks a far cry from his early activist days, when people yelled at him to cut his hair and find a job.

As an undergrad in 1970, he and a friend set up a table in the parking lot of the old White City Shopping Plaza in Shrewsbury to collect signatures for a petition to impeach President Richard Nixon.

“People were screaming at us: ‘If you don’t like the country then leave,’ ” said Sheehan. Things turned even uglier. A few guys shoved them around, knocking over their table and sending the petition papers flying all over the parking lot. When the police arrived, the students got the blame.

“Back then you had to get out into the streets and bring your message to everyday people who needed to hear it — a lot like today,” said Sheehan, whose anti-Vietnam War credentials also included joining a sit-in at the Worcester draft board and attending the huge October 1969 War Moratorium protest in Washington.

Sheehan recalled the night the Vietnam War arrived in Worcester.

“I can remember being with my male friends in the dorm room and watching General Hershey draw birth dates out of a large glass bowl,” he said, referring to the head of the US draft. “I’ll never forget some of my friends, who had gotten very low numbers, quietly getting up and walking out because they knew they were going to end up going to Vietnam. And they did.”

Sheehan’s latest project is to teach world history through the study of nongovernmental organizations, known as NGOs. Eleventh – grade students will zero in on a particular interest — such as the environment, world hunger, or women’s rights — and create a course of study. The students must then choose from one of thousands of NGOs such as Amnesty International, the Prevention of Child Soldiers in Africa, Doctors Without Borders, or the One Campaign with Bono to wipe out poverty.

“The idea is to have students become experts in their NGO field and at the end of the year put on a fair for the school and network with others to try to solve problems without depending upon the government,” said Sheehan.

Sheehan’s office is filled with mementos from his travels. Tibetan prayer flags hang on the wall behind his computer; a piece of the Berlin Wall is displayed on one shelf of a bookcase, a photo of the slain Nepalese royal family on another; and a bowl filled with coins from around the world sits on his desk.